The invention relates to a circuit for precisely correcting positive and negative linearity errors of a voltage-excited bridge sensor by a technique utilizing a minimum amount of circuitry and a minimum number of external package leads.
Resistive bridge circuits, i.e., bridge sensors, have nonlinearities due to mismatches in values of the bridge circuit elements. Many bridge sensors are inherently non-linear. It is possible to compensate for such non-linearity by varying the bridge excitation voltage proportionally to the output unbalance signal of the bridge. The following equation represents the bridge excitation voltage V.sub.EXCITE : EQU V.sub.EXCITE =V.sub.EXCITE(0).+-.V.sub.BROUT.times.K.sub.LIN, (Equation 1)
where V.sub.BROUT is the bridge circuit output voltage, K.sub.LIN is a linearization constant, and V.sub.EXCITE(0) is an initial value of V.sub.EXCITE.
The uncorrected signal results in a non-linear curve for V.sub.BROUT, as indicated by curve A in FIG. 5. Curve B in FIG. 5 represents the usually parabolic relative non-linearity of the bridge transducer that results in the nonlinear output of the bridge circuit indicated by curve A. Curve C represents the non-linearity after correction or linearization by varying the excitation voltage V.sub.EXCITE, and curve D represents the corrected bridge output voltage obtained as a result of correcting the excitation voltage by means of a feedback circuit coupled between the bridge output and V.sub.EXCITE.
A very effective technique for "linearizing" a bridge circuit is to modulate its "excitation source", i.e., the reference voltage which is applied to the bridge circuit. U.S. Pat. Nos. 4,190,796, 4,362,060, 4,492,122, 5,122,756 and 5,764,067 are illustrative of the state of the art. The known linearization circuits generally are used in conjunction with conventional instrumentation amplifiers which provide amplified outputs to suitable utilization circuits.
The above mentioned known linearization circuits generally require four external package leads to allow a user to determine both the polarity and magnitude of linearity corrections required for each individual bridge sensor circuit. However, the user often has no way of knowing in advance whether the polarity of linearity correction needed for a particular bridge sensor circuit is positive or negative. Consequently, the user may have to swap connections between two external leads of the bridge linearization circuit to get the correct polarity of linearization correction, which is inconvenient. Furthermore, it usually is undesirable to have to use more external package leads than is genuinely necessary, and it would be better to be able to adjust the magnitude of the needed correction with one, rather than two external package leads.
Accordingly, there is an unmet need for an improved bridge linearity correction technique which requires a reduced amount of circuitry and a reduced number of external package leads for setting both the polarity and magnitude of the linearity corrections required for each different bridge sensor.